1. Field of the Invention
This invention relates to electrically conductive articles and, more specifically, this invention relates to flexible, electrically conductive articles of polyurethane or similar plastic material.
2. Description of the Prior Art
Accumulation of static electrical charge in certain working environments has long been recognized as undesirable. Unfortunately, many widely used synthetic polymers, such as polyurethane and polyvinyl chloride, for example, are electrical insulators and articles of these materials thus may easily accumulate a substantial static electrical charge. Such an accumulation of charge may attract dust which may adversely affect the quality of manufactured products. Further, static electrical discharge may disturb the performance of electromagnetic machines, such as computers, in the vicinity of the discharge. In environments such as grain elevators, for example, fire or explosion may result from such discharge.
It has long been known that articles of electrically conductive material will, if grounded, dissipate static electricity as acquired. In the past, synthetic and natural polymers have been rendered electrically conductive by the incorporation of conductive particles therein. For example, particles of carbon black or copper powder may be mixed with a polymer before forming thereof into a desired article. Incorporation of particles of hygroscopic material into a polymer results in adsorption of moisture from the air onto the polymer surface to prevent the build-up of static charges.
However, particles mixed with a polymer tend to become encapsulated or otherwise separated from adjacent particles by the polymer. Therefore, relatively high particle loadings are required to effectively render the polymer electrically conductive. When a hygroscopic material is added to a polymer, control of the amount of moisture adsorbed on the polymer is very difficult or impossible, due to variability of the relative humidity of the air. In dry weather, insufficient moisture may be adsorbed to prevent static charge build-up.
Litant U.S. Pat. No. 3,406,126 issued Oct. 15, 1968, the disclosure of which is hereby incorporated by reference, discloses a method of rendering an article of synthetic resin electrically conductive by uniformly and randomly dispersing therethrough between about 0.05% and 5%, by weight, of carbon yarn fibers of between 1/4" and 3/4" in length, and about 10 microns in diameter. The fibers must have a very high length-to-diameter ratio (between about 640:1 and 1920:1) and have elastic recovery characteristics sufficient to assure that the fibers regain a linear configuration after distortion, such that fiber-to-fiber contact throughout the mass is assured. The fibers must be mixed with polymer in low shear machinery in order to prevent undesirable shortening of the fibers.
At low fiber loadings, it is necessary to mix a highly polar plasticizer with the polymer to enhance the article's conductivity. It is apparent from the Litant disclosure (see col. 10, lines 50-64) that, at fiber loadings of about 3 weight % or less, the resistivity of the article increases with decreasing fiber loadings to an undesirable level. The addition of a polar plasticizer is required to maintain resistivity below about 1,000 ohm-cm at fiber loading of 0.5%. This level is undesirably high for some applications.
Further, tests conducted on articles made according to the Litant disclosure indicate that conductivity of the articles decreases with repeated distortion of the articles, making such articles unsuitable for use in applications requiring continual or repeated flexing.